1. Field of the Invention
The invention relates generally to the field of geophysical surveying of the Earth's subsurface. More specifically, the invention relates to systems and methods for combined electromagnetic and seismic surveying of the Earth's subsurface.
2. Background Art
Techniques for geophysical surveying of the Earth's subsurface include seismic surveying and electromagnetic surveying. Seismic surveying includes deploying a seismic energy source at or near the Earth's surface above an area that is to be surveyed, and deploying one or more arrays of seismic sensors at or near the surface in the vicinity of the seismic energy source. The source is actuated at selected times and signals detected by the seismic sensors are recorded. Electromagnetic surveying includes deploying a plurality of electric and/or magnetic field sensors at or near the surface above an area to be surveyed and measuring electric and/or magnetic field amplitudes for a selected period of time. The measured electric and/or magnetic fields may result from electromagnetic radiation caused by naturally occurring phenomena (called “magnetotelluric” surveying) or may be in response to an electromagnetic field imparted into the rock formations beneath the surface (called “controlled source” surveying). Seismic and electromagnetic responses can be used to infer certain physical properties of the rock formations in the subsurface.
The foregoing geophysical surveying techniques are also used to survey formations below the bottom of a body of water such as a lake or the ocean. Such surveying, known as “marine” surveying, includes using a vessel to tow one or more cables in the water. Such cables can include a plurality of seismic sensors in the case of marine seismic surveying. A seismic energy source such as an air gun or arrays of such air guns can be deployed in the water. The seismic source is actuated and signals are detected, just as in the land based case described above. For marine electromagnetic surveying, the cable can include a plurality of electric and/or magnetic field sensors, and in the case of controlled source marine surveying, a cable including an electric and/or magnetic field generator can also be towed in the water.
Marine surveying can be particularly expensive because of the cost of operating the vessel used to tow the cables in the water. It is desirable to have methods and systems to perform marine seismic and electromagnetic surveying contemporaneously to reduce the cost of performing such surveying. It is also desirable to be able to combine seismic and electromagnetic field sensors into the same cable to reduce survey inaccuracies caused by differences in geodetic position of the sensors that inevitably occur when a survey system is deployed at different times over the same subsurface area. It is believed to be impracticable, however, to combine seismic sensing devices and electromagnetic sensing devices in a single sensor cable or streamer because seismic streamers must generally be operated at shallow water depth to reduce the effects on seismic signals from the subsurface of seismic energy reflecting from the water surface. Conversely, marine electromagnetic sensors must typically be operated at water depths believed to be unsuitable for seismic acquisition in order to reduce the effects of electromagnetic energy reflected from the water surface on electromagnetic signals from the subsurface.